Diesel aftertreatment systems

ABSTRACT

A method for improving NOx conversion efficiency of a NOx-reducing catalyst by determining an accurate amount of reductant required is presented. The method includes calculating an initial reductant injection amount based on a steady state amount of NOx in the engine feedgas and adjusting the initial amount to compensate for transient NOx emissions. The compensation is initiated in response to an engine transient, such as impending acceleration or deceleration. This method further results in improved vehicle fuel economy.

BACKGROUND OF INVENTION

[0001] 1. Field of Invention

[0002] The present invention relates to a system and a method forimproving performance of a NOx-reducing catalyst and, more particularly,to controlling an amount of reductant injection to achieve optimum NOxconversion efficiency while minimizing the fuel economy penalty.

[0003] 2. Background of the Invention

[0004] Current emission control regulations necessitate the use ofcatalysts in the exhaust systems of automotive vehicles in order toconvert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides(NOx) produced during engine operation into harmless exhaust gasses.Vehicles equipped with diesel or lean gasoline engines offer thebenefits of increased fuel economy. Such vehicles have to be equippedwith lean exhaust aftertreatment devices, such as, for example, anActive Lean NOx Catalysts (ALNC) or Selective Catalytic Reduction (SCR)catalysts, which continuously reduce NOx emissions, even in an oxygenrich environment, through active injection of reductant, such as fuel(HC) or urea, into the exhaust gas entering these devices. Further, itis important to precisely control the amounts of reductant in order toachieve maximum NOx conversion efficiency.

[0005] The inventors herein have recognized that transient changes inengine operating conditions cause changes in engine feedgas NOxproduction. For example, NOx production usually increases during engineacceleration, and decreases during deceleration. Since the amount ofreductant injection is typically calculated based on steady state engineoperating conditions, these transient NOx variations result in over orunder-injection of reductant and negatively impact fuel economy andemission standards.

SUMMARY OF INVENTION

[0006] In accordance with the present invention, a system and a methodfor controlling an amount of reductant to be delivered to a NOx-reducingcatalyst are presented. The method includes calculating a desired amountof reductant based on a measure of engine transient behavior; andinjecting said calculated desired amount of reductant into theNOx-reducing catalyst.

[0007] In one aspect of the present invention, the device is an ALNC andthe reductant is hydrocarbon. In another aspect of the presentinvention, the device is an SCR catalyst and the reductant is urea. Inyet another aspect of the present invention, the measure of enginetransient behavior is a measure of engine acceleration. In anotheraspect of the present invention, the measure further includes enginedeceleration. In another aspect of the present invention, the measure ofengine transient behavior is based on a rate of change of pedalposition. In yet another aspect of the present invention, the measure ofengine transient behavior is based on a rate of change of engine fuelinjection amount. In yet another aspect of the present invention, themeasure of engine transient behavior is based on a rate of change ofengine speed.

[0008] In another aspect of the present invention, a method forimproving efficiency of a NOx-reducing catalyst coupled downstream of aninternal combustion engine includes: providing an indication of animpending engine transient; and adjusting an amount of reductantinjection into the NOx-reducing catalyst to compensate for variations inengine feedgas NOx caused by said impending engine transient.

[0009] The present invention provides a number of advantages. Inparticular, NOx conversion efficiency of the NOx-reducing catalyst isimproved by adjusting the injected reductant amounts to compensate fortransient increases or decreases in the engine feedgas NOx amounts.Further, monitoring the rate of change of pedal position provides aquick and accurate indication of an impending engine transient and theassociated change in engine feedgas NOx. Thus, reductant injectionamount can be timely adjusted to compensate for NOx variations. Anotheradvantage of the present invention is improved fuel economy due tooptimized reductant usage. For example, reductant injection amount canbe reduced in anticipation of engine deceleration to compensate for areduction in engine feedgas NOx.

[0010] The above advantages and other advantages, objects and featuresof the present invention will be readily apparent from the followingdetailed description of the preferred embodiments when taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The objects and advantages described herein will be more fullyunderstood by reading an example of an embodiment in which the inventionis used to advantage, referred to herein as the Description of PreferredEmbodiment, with reference to the drawings, wherein:

[0012]FIGS. 1A and 1B are schematic diagrams of an engine wherein theinvention is used to advantage;

[0013]FIG. 2 is an example of a reductant delivery system used toadvantage with the present invention;

[0014]FIGS. 3 and 4 describe an exemplary routine and a modificationcurve for determining an amount of reductant to be delivered to theexhaust gas aftertreatment device in accordance with the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS(S)

[0015] Internal combustion engine 10, comprising a plurality ofcylinders, one cylinder of which is shown in FIG. 1A, is controlled byelectronic engine controller 12. Engine 10 includes combustion chamber30 and cylinder walls 32 with piston 36 positioned therein and connectedto crankshaft 40. Combustion chamber 30 is shown communicating withintake manifold 44 and exhaust manifold 48 via respective intake valve52 and exhaust valve 54. Intake manifold 44 is also shown having fuelinjector 80 coupled thereto for delivering liquid fuel in proportion tothe pulse width of signal FPW from controller 12. Both fuel quantity,controlled by signal FPW and injection timing are adjustable. Fuel isdelivered to fuel injector 80 by a fuel system including a fuel tank,fuel pump, and fuel rail (not shown).

[0016] Controller 12 is shown in FIG. 1A as a conventional microcomputerincluding:

[0017] microprocessor unit 102, input/output ports 104, read-only memory106, random access memory 108, and a conventional data bus. Controller12 is shown receiving various signals from sensors coupled to engine 10,in addition to those signals previously discussed, including: enginecoolant temperature (ECT) from temperature sensor 112 coupled to coolingsleeve 114; a measurement of manifold pressure (MAP) from pressuresensor 116 coupled to intake manifold 44; a measurement (AT) of manifoldtemperature from temperature sensor 117; an engine speed signal (RPM)from engine speed sensor 118 coupled to crankshaft 40.

[0018] Oxidation catalyst 13 is coupled to the exhaust manifold 48downstream of engine 10 and may be a precious metal catalyst, preferablyone containing platinum. Catalyst 14, a NOx-reducing catalyst capable ofreducing NOx in an oxygen rich environment, is coupled downstream of theoxidation catalyst. In a preferred embodiment Catalyst 14 is an ActiveLean NOx Catalyst (ALNC) comprising a precious metal or a combination ofprecious metals, such as Platinum or Palladium, an acidic supportmaterial, such as the one containing alumina and silica, and a zeolitematerial. In an alternative embodiment, catalyst 14 may be a urea-basedSelective Catalytic Reduction (SCR) catalyst, which is a devicecomprising some or all of the features of the ALNC catalyst andoptimized for use with urea or other ammonia-based compounds asreductant. The oxidation catalyst 13 exothermically combustshydrocarbons (HC) in the incoming exhaust gas from the engine thussupplying heat to rapidly warm up catalyst 14. Additionally, carbonmonoxide (CO) produced as a result of HC combustion in the oxidationcatalyst 13 improves NOx reduction in the catalyst 14.

[0019] A reductant delivery system 16 is coupled to the exhaust gasmanifold between the oxidation catalyst and the NOx-reducing catalystand is described in more detail in FIG. 2 below. Alternatively,reductant delivery system 16 may be any system known to those skilled inthe art capable of supplying reductant to the NOx-reducing catalyst. Ina preferred embodiment, reductant delivery system injects fuel(hydrocarbon) into the exhaust gas mixture entering catalyst 14.Alternatively, reductant delivery system 16 may supply aqueous urea tothe NOx-reducing catalyst.

[0020] Referring now to FIG. 1B, an alternative embodiment is shownwhere engine 10 is a direct injection engine with injector 80 located toinject fuel directly into cylinder 30.

[0021] The diagram of FIG. 2 generally represents an example of oneembodiment of a reductant delivery system according to the presentinvention. The system comprises an evaporator unit 21 housing anelongated heating element 22. The mixing unit 23 has a reductant inletand an air inlet and an outlet 24 coupled to the evaporator unit 21through which a mixture of reductant and air is injected into thehousing and subsequently comes into contact with the surface of theheating element 22. Alternatively, both air and reductant can beinjected through a single input. The reductant can be supplied to themixing unit 23 from the fuel tank or from a storage vessel. Air pump 25supplies pressurized air to the mixing unit 23 thereby creating amixture of reductant and air. Outlet 24 is configured to deliver thereductant and air mixture to more than one area on the surface of theheating element. Controller 12 can selectively enable and disableinjection of the mixture to these areas depending on operatingconditions, such as engine speed, load, exhaust gas temperature, etc.For example, when the amount of reductant required is high, such as athigh load conditions, it may be necessary to enable delivery of thereductant and air mixture to more than one area on the surface of theheating element. Alternatively, outlet 24 may be configured to deliverthe reductant and air mixture to a specific area on the surface of theheating element.

[0022] As will be appreciated by one of ordinary skill in the art, theroutines described in FIGS. 3 and 4 below may represent one or more ofany number of processing strategies such as event-driven,interrupt-driven, multi-tasking, multi-threading, and the like. As such,various steps or functions illustrated may be performed in the sequenceillustrated, in parallel, or in some cases omitted. Likewise, the orderof processing is not necessarily required to achieve the objects,features and advantages of the invention, but is provided for ease ofillustration and description. Although not explicitly illustrated, oneof ordinary skill in the art will recognize that one or more of theillustrated steps or functions may be repeatedly performed depending onthe particular strategy being used.

[0023] Referring now to FIG. 3, an exemplary routine for controllinginjection of a reductant into exhaust flow is presented. First, in step500, the amount of NOx in the exhaust gas mixture entering the device,NOx_(fg), is estimated based on engine operating conditions. Theseconditions may include engine speed, engine load, exhaust temperatures,exhaust gas aftertreatment device temperatures, injection timing, enginetemperature, and any other parameter know to those skilled in the art toindicate the amount of NOx produced by the combustion presses.Alternatively, a NOx sensor may be used to measure the amount of NOx inthe exhaust gas mixture. Next, in step 600, the steady-state reductantinjection amount, RA_(inj) _(—) ₁, is calculated based on the followingequation:$\frac{\left( {{RA}_{fg} + {RA}_{{inj\_}1}} \right)}{{NOx}_{fg}} = R_{des}$

[0024] wherein

RA_(fg)

[0025] is the amount of reductant in the engine feedgas, which can bedetermined based on engine operating conditions. This initial reductantamount,

RA_(inj) _(—) ₁,

[0026] is evaluated at steady state and yields a base reductant quantityto be injected for each engine speed and load point. The amount iscalibrated to achieve a certain feedgas reductant to NOx ratio, R_(des).The ratio is typically obtained as a trade-off between NOx conversionand the fuel penalty due to reductant injection, and in this example isset at approximately 10. Next, in step 700, the steady-state basereductant injection amount,

RA_(inj) _(—) ₁,

[0027] is modified to account for engine operating conditions, such asengine coolant temperature,

T_(e),

[0028] exhaust gas temperature,

T_(eg),

[0029] EGR valve position,

EGR_(pos),

[0030] start of injection,

SOI,

[0031] and other parameters:

RA _(inj) _(—) ₂ =RA _(inj) _(—) ₁ ·f ₁(T _(c))·f ₂(T _(eg))·f ₃(SOI)·f₄(EGR _(pos))

[0032] The routine then proceeds to step 800 wherein the rate of changeof pedal position is computed as follows:${{pps\_ diff}(t)} = \frac{\left( {{{pps}(t)} - {{pps}\left( {t - 1} \right)}} \right)}{T_{s}}$

[0033] where T_(S) is the sampling rate, and

pps(t)

[0034] denotes the pedal position at time

t.

[0035] Next, in step 900, a low pass filter is applied to smooth out thenoise:

pps_diff_(—) lp(t)=(1−k _(f))·pps_diff_(—) lp(t−1)+k _(f) ·pps_diff(t−1)

[0036] where

k_(f)

[0037] controls the rate of filtering. The routine then proceeds to step1000 wherein the reductant amount is further modified to account forengine transient behaviors as represented by the changes in the pedalposition:

RA _(inj 3) =RA _(inj 2) ·f ₅(pps_diff_(—) lp)

[0038] where function

f₅

[0039] is shaped to allow overinjection of reductant during pedalposition tip-in and underinjection of reductant during pedal positiontip-out. An example of

f₅

[0040] is shown with particular reference to FIG. 6. In an alternativeembodiment, rate of change of engine speed, rate of change of enginefuel injection amount, rate of change of engine load, rate of change ofengine fuel demand or any other parameter known to those skilled in theart to provide a measure of engine transient behavior may be used toobtain

RA_(inj) _(—) ₃.

[0041] The routine then exits.

[0042] In an alternative embodiment (not shown), the modifiedsteady-state reductant injection amount,

RA_(inj 2).

[0043] calculated in step 700, is further modified to account for enginetransient behavior only if the rate of change of pedal position isgreater than a predetermined calibratable value.

[0044] Therefore, according to the present invention, in order toachieve a more efficient NOx-reducing catalyst performance, the amountof reductant to be injected should be adjusted to account for increasesand decreases in the amount of NOx in the exhaust gas entering thecatalyst. This can be accomplished by continuously monitoring engineparameters that are capable of providing a measure of engine transientbehaviors, and continuously adjusting the amount of reductant to beinjected as a function of these parameters. Since NOx productiontypically increases at tip-in and decreases at tip-out, the result ofsuch operation would be to increase thee base injected amount in theformer case, and decrease the base injected amount in the latter case.By monitoring parameters that are capable of providing very quickindication of engine transients, such as, for example, rate of change ofpedal position, rate of change of engine fuel injection amount, or rateof change of engine speed or load, it is possible optimize systemresponse and ensure that optimal reductant amount is timely injectedinto the device in response to a change in engine feedgas NOx.

[0045] This concludes the description of the invention. The reading ofit by those skilled in the art would bring to mind many alterations andmodifications without departing from the spirit and the scope of theinvention. Accordingly, it is intended that the scope of the inventionbe defined by the following claims:

1. A method for controlling a NOx-reducing catalyst coupled downstreamof an internal combustion engine, comprising: calculating a desiredamount of reductant based on a measure of engine transient behavior; andinjecting said calculated desired amount of reductant into the leanexhaust gas aftertreatment device.
 2. The method as set forth in claim 1wherein the engine is a diesel engine.
 3. The method as set forth inclaim 1 wherein the NOx-reducing catalyst is an ALNC.
 4. The method asset forth in claim 3 wherein said reductant is hydrocarbon.
 5. Themethod as set forth in claim 1 wherein the NOx-reducing catalyst is anSCR catalyst.
 6. The method as set forth in claim 5 wherein saidreductant is urea.
 7. The method as set forth in claim 1 wherein saidmeasure of engine transient behavior comprises a measure of enginedeceleration.
 8. The method a set forth in claim 1 wherein said measureof engine transient behavior is based on a rate of change of pedalposition.
 9. The method as set forth in claim 1 wherein said measure ofengine transient behavior is based a rate of change of engine speed. 10.The method as set forth in claim 1 wherein said measure of enginetransient behavior is based a rate of change of engine fuel injectionamount.
 11. The method as set forth in claim 1 wherein said measure ofengine transient behavior is based a rate of change of engine load. 12.The method as set forth in claim 1 wherein said measure of enginetransient behavior is filtered.
 13. A method for controlling aNOx-reducing catalyst coupled downstream of an internal combustionengine, comprising: calculating an initial reductant amount; adjustingsaid initial reductant amount to compensate for engine transientbehavior; and injecting said adjusted initial amount of reductant intothe exhaust gas aftertreatment device.
 14. The method as set forth inclaim 13 wherein the NOx-reducing catalyst is an ALNC.
 15. The method asset forth in claim 14 wherein said reductant is hydrocarbon.
 16. Themethod as set forth in claim 13 wherein the NOx-reducing catalyst is anSCR catalyst.
 17. The method as set forth in claim 16 wherein saidreductant is urea.
 18. The method as set forth in claim 13 wherein theengine is a diesel engine.
 19. The method as set forth in claim 13wherein said initial amount of reductant is based on a steady stateestimate of an amount of NOx in an engine exhaust gas.
 20. The method asset forth in claim 13 wherein said engine transient behavior is measuredby calculating a rate of change of pedal position.
 21. The method as setforth in claim 13 wherein said engine transient behavior is measured bycalculating a rate of change of engine fuel injection amount.
 22. Themethod as set forth in claim 13 wherein said engine transient behavioris measured by calculating a rate of change of engine speed.
 23. Themethod as set forth in claim 13 wherein said engine transient behavioris measured by calculating a rate of change of engine load.
 24. A systemfor reducing NOx emissions in an engine exhaust gas mixture, comprising:a NOx-reducing catalyst coupled downstream of the engine; and acontroller injecting reductant into said NOx-reducing catalyst whereinsaid amount of injected reductant is based on a measure of enginetransient behavior.
 25. A method for improving efficiency of a NOxreducing catalyst coupled downstream of an internal combustion engine,comprising: providing an indication of an impending engine transient;and adjusting an amount of reductant injection into the NOx-reducingcatalyst to compensate for variations in engine feedgas NOx caused bysaid engine transient.
 26. A system for reducing transient andsteady-state NOx emissions in the exhaust gases of a vehicle powered bya diesel fueled internal combustion engine comprising: a) a NOx-reducingcatalyst downstream of said engine; b) a source of liquid hydrocarbons;c) a valve for introducing predetermined quantities of said hydrocarbonsfrom said source into said exhaust gases upstream of said NOx-reducingcatalyst pursuant to a command signal; d) a plurality of vehicle sensorsgenerating sensor signals indicative of at least one operating conditionof said engine, said sensors including at least one transient operationsensor generating a signal prior to a transient operating condition ofsaid engine and predictive of said transient operating condition; e) anengine control unit having a plurality of programmed routines forcontrolling said engine in response to said plurality of sensor signals,said routines including at least (1) a first routine for generating afirst command signal for introducing a first predetermined quantity ofsaid hydrocarbons through said valve when said engine is operating at asteady state condition sufficient to reduce a portion of NOx emissionsproduced at the steady state condition, and (2) a second routineactivated when said at least one transient operating sensor generatessaid signal predictive of said transient operating condition, saidsecond routine comprising: i) calculating a second quantity ofhydrocarbons sufficient to reduce a portion of the NOx emissionsgenerated during a period in which said engine is operating in saidtransient operating condition and ii) generating a second command signalfor introducing said second predetermined quantity of said hydrocarbonsthrough said valve during at least a portion of the period in which saidengine is operating in said transient operating condition.
 27. Thesystem of claim 26 wherein said source of hydrocarbons comprises adiesel fuel tank of said vehicle.
 28. The system of claim 26 whereinsaid transient operation sensor comprises an engine speed sensor. 29.The system of claim 26 wherein said transient operation sensor comprisesa pedal position sensor.
 30. The system of claim 26 wherein saidtransient operation sensor is indicative of a change in speed of saidengine.
 31. The system of claim 26 wherein said transient operationsensor is indicative of deceleration of said engine.
 32. The system ofclaim 26 wherein said transient operation sensor is indicative of achange in load of said engine.
 33. The system of claim 26 wherein saidtransient operating condition is controlled by the operator of saidvehicle.
 34. A method for reducing NOx emissions present in the exhaustgases of an internal combustion engine comprising the steps of: a)providing a source of liquid hydrocarbons; b) providing a reducingcatalytic converter downstream of said engine through which said exhaustgases pass; c) sensing a set of engine operating parameters including atleast a space velocity of the exhaust gases and a temperature of theexhaust gases; d) calculating a first concentration of hydrocarbon andNOx emissions present in said exhaust gases based on said sensed engineoperating parameters during a steady state operation of said engine; e)determining an amount of said hydrocarbons sufficient to reduce adesired percentage of said NOx emissions when said exhaust gases leavesaid reducing catalytic converter during said steady state operation ofsaid engine; f) metering a quantity of said hydrocarbons into saidexhaust gases upstream of said reducing catalytic converter sufficientto reduce said desired percentage of said NOx emissions during saidsteady state operation of said engine; g) sensing a signal predictive ofa transient operation state of said engine; h) calculating the expectedconcentration of NOx emissions present in said exhaust gases while theengine is in said transient operation state based on said signalpredictive of said transient operation state; and i) metering anadditional quantity of said hydrocarbons into said exhaust gasesupstream of said reducing catalytic converter sufficient to reduce saidexpected concentration of NOx emissions during said transient engineoperation state.
 35. The method of claim 34 in which said internalcombustion engine is a lean operating gasoline engine.
 36. The method ofclaim 34 wherein said lean operating gasoline engine operates at anair-to-fuel ratio of at least 1.03.
 37. The method of claim 34 whereinsaid signal predictive of a transient operating state of said engineindicates a change in engine fuel demand.
 38. The method of claim 34wherein said signal predictive of a transient operating state of saidengine indicates a change in engine fueling rate.
 39. The method ofclaim 34 wherein said signal predictive of a transient operating stateof said engine indicates a change in a pedal position.
 40. The method ofclaim 34 wherein said signal predictive of a transient operating stateof said engine indicates a change in engine speed.
 41. The method ofclaim 34 wherein said signal predictive of a transient operating stateof said engine indicates a change in engine load.
 42. The method ofclaim 34 wherein said signal predictive of a transient operating stateof said engine indicates deceleration of said engine.
 43. The method ofclaim 34 wherein said transient operating state of said engine isresponsive to an operator inputted command.
 44. The method of claim 34wherein said sensed operating conditions include the air-to-fuel ratioof the engine.
 45. A method for reducing NO_(x) emissions produced by avehicle powered by a diesel engine having an active lean NOx catalystthrough which the exhaust gases pass and sensors for determining NO_(x)concentrations at steady state engine operating conditions, theimprovement comprising the steps of: a)sensing a pedal position sensorsignal prior to an engine transient; and b)injecting a set quantity ofdiesel fuel reductant into the exhaust gases of the engine sufficient tosubstantially react with transient emissions produced by the engineduring said transient, the injection occurring at a time in advance ofand not later than the time the engine produces the transient emissions.